The field of the invention is nuclear magnetic resonance imaging methods and systems. More particularly, the invention relates to the production of images in a fast cardiac MRI acquisition.
When a substance such as human tissue is subjected to a uniform magnetic field (polarizing field B.sub.0), the individual magnetic moments of the spins in the tissue attempt to align with this polarizing field, but precess about it in random order at their characteristic Larmor frequency. If the substance, or tissue, is subjected to a magnetic field (excitation field B.sub.1) which is in the x-y plane and which is near the Larmor frequency, the net aligned moment, M.sub.z, may be rotated, or "tipped", into the x-y plane to produce a net transverse magnetic moment M.sub.t. A signal is emitted by the excited spins after the excitation signal B.sub.1 is terminated and this signal may be received and processed to form an image.
When utilizing these signals to produce images, magnetic field gradients (G.sub.x G.sub.y and G.sub.z) are employed. Typically, the region to be imaged is scanned by a sequence of measurement cycles in which these gradients vary according to the particular localization method being used. The resulting set of received NMR signals are digitized and processed to reconstruct the image using one of many well known reconstruction techniques.
Most NMR scans currently used to produce medical images require many minutes to acquire the necessary data. The reduction of this scan time is an important consideration, since reduced scan time increases patient throughput, improves patient comfort, improves image quality by reducing motion artifacts, and enables the performance of medical test procedures such as timed pharmacological stress tests (e.g. multi-stage dobutamine stress test). There is a class of pulse sequences which have a very short repetition time (TR) and result in complete scans which can be conducted in seconds rather than minutes. When applied to cardiac imaging, for example, a complete scan from which a series of images showing the heart at different phases of its cycle or at different slice locations can be acquired in a single breath-hold.
There are two common techniques for acquiring cardiac MR images. The first is a prospectively gated, single-phase, multi-slice conventional spin echo sequence. In each cardiac cycle, data at different spatial locations are acquired with the same k-space phase encoding value. Images at the different spatial locations are then acquired at different temporal phases of the cardiac cycle. Since only one k-space line is acquired per cardiac trigger, a typical scan with a 128 k-space views in the phase encoding direction will take 128 heart beats to complete. The sequence repetition time (TR) is then the cardiac R--R interval time.
A short TR gated gradient echo pulse sequence may be used to acquire (CINE) images at multiple time frames of the cardiac cycle. As described in U.S. Pat. No. 4,710,717, conventional CINE pulse sequences run asynchronously to the cardiac cycle with the phase encoding value stepped to a new value at each R-wave trigger. In CINE, each rf excitation pulse is applied at the same spatial location and repeated at intervals of TR in the cardiac cycle. Since the sequence runs asynchronously, the rf excitation pulses may occur at varying time delays from the R-wave from one cardiac cycle to the next. On detection of the next cardiac R-wave, the acquired data from the previous R--R interval are resorted and interpolated into evenly distributed time frames within the cardiac cycle. This method of gating is also known as retrospective gating as the data for the previous R--R interval is resorted only after the current R-wave trigger is detected.
As in gated spin echo, only one k-space phase encoding view is acquired per heart beat. The total image acquisition time is then on the order of 128 heart beats.
Faster scan times can be achieved by segmenting k-space and acquiring multiple phase encoding k-space views per R--R interval. The scan time is speeded up by a factor equal to that of the number of k-space views acquired per image per R--R interval. In this manner, a typical CINE acquisition with a matrix size of 128 pixels in the phase encoding direction can be completed in as little as 16 heart beats, when 8 k-space views per segment are acquired.
Multiple phases of the cardiac cycle can be visualized by repeated acquisition of the same k-space segment within each R--R interval but assigning the data acquired at different time points in the cardiac cycle to different cardiac phases. Thus, the cardiac cycle is sampled with a temporal resolution equal to the time needed to acquire data for a single segment, such that EQU temporal resolution=vps.times.TR,
where vps is the number of k-space lines per segment, the TR is the pulse sequence repetition time. The total scan time is then ##EQU1## where yres is the number of phase encoding views in the image. Typically, an image utilizes 128 or more phase encoding views, and 8 views per segment is also often used.
In segmented k-space scans, the total scan time can be substantially reduced by increasing the number of views per segment (vps). However, this is at the expense of reducing the image temporal resolution. As described in U.S. Pat. No. 5,377,680 the image temporal resolution can be increased by sharing views between adjacent time segments to generate images averaged over different time points. The true image temporal resolution is unchanged but the effective temporal resolution is now doubled. View sharing can thus increase the number of cardiac phase images reconstructed without affecting the manner in which the k-space data is acquired.
Prospectively gated, segmented k-space sequences have become popular for cardiac imaging mainly because images can be obtained in a breath-hold and therefore do not suffer from respiratory artifact. Images are formed by acquiring data over a series of heartbeats with data acquisition gated to the QRS complex of the ECG. The assessment of cardiac function is typically performed using an ECG-gated segmented k-space fast gradient echo (FGRE) pulse sequence. Using this approach, multi-phase images of the heart with approximately 40-80 ms temporal resolution are typically acquired in 10-25 seconds, usually during suspended respiration (breathholding). In each heartbeat a segment is played out repeatedly (at multiple cardiac phases), where each segment consists of N.sub.vps FGRE pulse sequence repetitions, where N.sub.vps is the number of views per segment. Within a segment, each FGRE pulse sequence repetition acquires one distinct line of data (a view), so that N.sub.vps different views are acquired per segment. If, for example, 128 views are required per image, and N.sub.vps =8, and TR=10 ms, then 16 heartbeats (approximately 16 seconds) are needed to acquire a complete set of multi-phase cardiac images at each heart location, and the nominal temporal resolution of each image is 80 ms. In practice, typically 12-15 locations covering the entire heart in the short-axis orientation are scanned. Therefore, using a conventional segmented k-space FGRE pulse sequence, 12-15 breathholds, each with an approximately 16 second duration, are required. This amount of breathholding can be excessive for some patients with heart disease, and may be prohibitive for meeting the timing needs of functional cardiac stress testing.